1. Field of the Invention
The present invention relates to spark plugs for internal combustion engines and, more particularly, to a spark plug for an internal combustion engine of a motor vehicle or the like.
2. Description of the Related Art
In the related art, attempts have heretofore been made to provide spark plugs as igniting means for internal combustion engines of motor vehicles or the like.
The spark plug generally includes a center electrode and a ground electrode between which a spark discharge gap is provided. Applying a high voltage across the center electrode and the ground electrode allows a spark discharge to take place in the spark discharge gap, thereby igniting an air-fuel mixture.
With the development of automotive internal combustion engines operating on high fuel consumption at high power output, there's been an increase of research into direct injection (DI) engines. The DI engines are generally categorized by combustion modes in two types including a stratified charge combustion type and a homogeneous and stoichiometric combustion. In any case, issues arise with spark plugs suffering from the occurrence of defacements due to fuel directly injected into combustion chambers. Therefore, in recent years, it has been strongly desired to provide new types of spark plugs having increased defacement resistance.
That is, with the DI engine of the related art discussed above, as fuel is directly injected into the combustion engine to form an air/fuel mixture therein and fuel in the air/fuel mixture is liable to adhere onto a center electrode 16 and a ground electrode 18 as shown in FIG. 6. Then, there is a fear of fuel progressively adhering onto the center electrode 16 and the ground electrode 18 so as to link these electrodes in a fuel bridge 30. As the fuel bridge 30 is created, even if a spark discharge is performed, a misfiring tends to occur due to anti-inflammatory action resulting from the presence of liquid fuel. This results in the occurrence of a fear of deterioration occurring in startability of the internal combustion engine.
To address such issues, Japanese Patent Unexamined Application Publication No. 2001-307858 discloses a spark plug structured to suppress the occurrence of a fuel bridge by forming a center electrode in a narrow diameter or utilizing means such as a protruding portion provided on a ground electrode.
However, even with such measure taken on the spark plug, the spark plug encounters a difficulty of appropriately removing the fuel bridge from the center electrode and the ground electrode under circumstances where a large amount of fuel is present for the fuel bridge to be formed like a situation where the engine remains at, for instance, ultracold temperatures or circumstances where fuel has a high viscosity.